Device for producing laser light

ABSTRACT

In a device to generate laser light, with a laser-active main resonator ( 2 ) and a coupled non-linear resonator ( 6 ), each of the two resonators ( 2, 6 ) being delimited by mirrors ( 8, 4; 4, 10 ), and one mirror ( 4 ) of the main resonator ( 2 ) being identical to a mirror ( 4 ) of the coupled non-linear resonator ( 6 ), it is provided that the non-linear resonator ( 6 ) is designed for a power loss less than 63% of the power which is coupled via the main resonator ( 2 ) and the mirror which delimits the coupled non-linear resonator ( 6 ), and the laser-active main resonator is designed so that in laser operation it has a threshold pump power which is less than one fifth, preferably less than one tenth, of the pump power which is used and using which this resonator is excited.

[0001] The invention concerns a device to generate laser light, with alaser-active main resonator and a coupled non-linear resonator. Each ofthe two resonators is delimited by mirrors, and one mirror of the mainresonator is identical to a mirror of the coupled non-linear resonator.

[0002] In the case of a resonator of a device to generate laser light,several modes of laser oscillation usually occur. Particularly in thecase of pulsed lasers in the picosecond range, which because of theshortness of the wave train emit a very large wavelength spectrum, thiscan result in instabilities, since with each pulse statistically adifferent amplitude distribution with reference to the different modescan be received.

[0003] This can be avoided with various methods of mode locking. Modelocking means that the longitudinal modes of a laser have a fixed phaserelationship, and are thus rigidly coupled to each with respect tophase. The effect of this is above all that the laser emits its energyin the form of ultra-short pulses. Depending on the laser material andthe type of mode locking, these ultra-short pulses have pulse durationsfrom about 100 ps to a few femtoseconds. The time gap from oneultra-short pulse to the next pulse corresponds to the resonator cycletime.

[0004] The various known methods can be divided into active and passivemethods. The invention is explained below exclusively on the basis ofpassive methods.

[0005] In the case of passive mode locking, a pulse which cycles in theresonator itself modulates its amplitude. Active components foramplitude or frequency modulation can then be omitted.

[0006] For passive mode locking, a non-linear element, the effect ofwhich can be most simply understood on the basis of the saturableabsorption, is used which the absorption is saturated at high lightintensities and the transmission is thus increased compared with lowlight intensities. In this way preferably a state of higher lightintensity is forced. This is achieved above all with the resultingshortening of a pulse, which is thus set as a reproducible physicalstate. The saturated absorption can be implemented, for instance, usingsemiconductor components, in which case the optical excitation enableselecrtons to switch completely from the valence band to the conductionband with suitably high intensity.

[0007] However, what is also particularly important here is thenon-linear phase shift resulting from the mutual effect of the laserlight and a medium which has a non-linear index of refraction,particularly because of the Kerr effect. Self phase modulation of thelight occurs if the Kerr non-linearity causes a time-dependent phaseshift, for instance in an optical fibre. Additionally, self-focusing ofthe laser light is possible if the Kerr non-linearity causes alocation-dependent phase shift. Such behaviour can be describedtheoretically analogously to the saturable absorber which is givenabove, by a so-called artificial saturable absorber.

[0008] Physically, it makes a big difference whether the process isresonant or non-resonant. Resonant processes run more slowly thannon-resonant ones. The optical Kerr non-linearity effect which is givenabove as an example is a non-resonant process of only a few femtosecondsrise time. Thus fast modulation is possible, so that essentially norecovery time has to be taken into account.

[0009] Because of the non-resonant mutual effect, mode locking ispossible in particular over a wide wavelength range. Above all in thecase of the Kerr process with an optical fibre, the absorption is alsovery low, and can be increased as required by providing a suitablelength of fibre. In this way, simple adjustment of the lasers bysuitable choice of the parameters of the non-linear resonator becomespossible, so that mode locking is achievable with different lasersystems and lasers with different output data, particularly maximumoutput power, pulse duration and repetition rate of the pulses.

[0010] For such mode locking, an auxiliary resonator to which anon-linear resonator is coupled is usually used, as described, forinstance, in U.S. Pat. No. 5,054,027. The coupling is usually via aninterferometer. However, this causes mode locking to become expensiveand liable to faults. Above all, because of the necessary precisepositioning of an interferometer to fractions of a wavelength, speciallystable construction is required.

[0011] However, the specified US patent specification also shows as analternative a simpler arrangement, in which the mirror of the mainresonator, which is opposite the mirror to extract the generated laserlight, is equal to one of the mirrors of the non-linear resonator. Totransmit the light from the main resonator into the non-linearresonator, this mirror is partially translucent, with a transmission of5-35%.

[0012] However, some tests have shown that with the specified values nopermanently stable mode operation was possible. This embodiment isprobably given in the US patent specification rather as an idea, thepractical implementation of which is probably not immediately possible.One therefore continues to depend on the known interferometric methods,with the stated disadvantages.

[0013] In this connection, constructions with Fabry-Perot resonators orMichelson interferometers are known. For the special configurations,reference is made to the extensive literature.

[0014] A further disadvantage, which limits these methods to systemswith comparatively low output power of less than 7 W, is the necessityof coupling the laser light into a single-mode optical fibre, which hasonly a short lifetime with high coupled powers.

[0015] On the other hand, there is a high requirement for mode-lockedlaser beam sources, which supply ultra-short pulses of less than 20 psinto the femtosecond range. As well as metrology for capturing veryprecise time flows, above all projection of pictures, particularly videopictures, using lasers should be mentioned here. Above all, there is arequirement because it is desirable to make large picture televisiontechnology possible for the consumer field.

[0016] A laser supplies a sufficiently high light density for thispurpose. However, irrespective of the imaging technology which is used,because of the high coherence of laser light undesired glitter effectsoccur. They can be effectively avoided by a large spectral width or asmall coherence length. With ultra-short pulses, this property is givenfrom the outset.

[0017] The following features are therefore extremely desirable for alaser source which can be used in this way:

[0018] a high average power of a few watts or more;

[0019] a short pulse duration of the laser pulses, of less than 20 ps;

[0020] a high repetition rate of the laser pulses, of more than 40 MHz;

[0021] a good beam quality, with diffraction limited as far as possible;

[0022] as simple and compact construction as possible;

[0023] use of a mode locking method which can be used for differentlaser materials and emission wavelength ranges;

[0024] robustness against external interference and adjustments of theparameters of the laser or resonator; and

[0025] mode locking made possible for different repetition rates,without significant restriction.

[0026] Because of the above-mentioned disadvantages, it cannot beexpected that all these aims can be achieved simply. This invention wastherefore based on a significantly simpler problem, which fulfils theessential requirements for increased stability and robust construction.

[0027] The object of the invention is to create a device which hasrobust construction and can be operated stably over time, to generate apulsed laser beam using mode locking.

[0028] On the basis of the prior art as stated in the introduction, theobject is achieved in that the non-linear resonator is designed for apower loss less than 63% of the power which is coupled via the mainresonator and the mirror which delimits the non-linear resonator.Additionally, the laser-active main resonator must be designed so thatin laser operation it has a threshold pump power which is less than onefifth, preferably less than one tenth, of the pump power which is usedand using which the main resonator is excited.

[0029] The ratio of the pump power in laser operation and the thresholdpump power is the threshold magnification. The threshold magnificationcan be determined very easily from the frequency of the relaxationoscillations, i.e. the frequency with which the output power oscillatesaround the equilibrium position, and the total losses of the mainresonator.

[0030] H. G. Danielmeyer, “Low-frequency dynamics of homogeneousfour-level-cw lasers”, Journal of Applied Physics 41, (1970) 4014applies:

r=1+4n ² f _(R) ² τ _(Sp) T _(R)/(2α _(r))

[0031] where r is the threshold magnification, fR is the frequency ofthe relaxation oscillations, 2α_(r) is the total losses in the mainresonator, T_(R) is the resonator cycle time, and t_(sp) is thefluorescence lifetime of the upper laser level.

[0032] As explained above, tests with the construction which is knownfrom U.S. Pat. No. 5,054,027, with a main resonator and a couplednon-linear resonator, each of the two resonators being delimited bymirrors and one mirror of the main resonator being identical to a mirrorof the coupled non-linear resonator, have shown no success withreference to the achievable stability.

[0033] On the other hand, it would be expected that this constructionwould allow a higher mechanical stability than with the knownconstructions with interferometers, with which in any case a positioningprecision of fractions of wavelengths is required.

[0034] However, it has been found unexpectedly with a special testconstruction that even if the transmission of the common mirror of themain resonator and non-linear resonator was less than 5%, and thereforequite different from the teaching of the US patent specification, stablemode-locked laser operation became possible. Further consideration andtests, as is shown in more detail below, have led to the conclusion thatthe essential feature which allows robust construction with stablemode-locked laser operation is not the transmission of the mirror butexclusively conformity to the specified upper limit of 63% for the powerloss in the non-linear resonator, and that the laser-active mainresonator in laser operation has a threshold pump power which is lessthan one fifth, preferably less than one tenth, of the pump power whichis used and using which the laser-active main resonator is excited.

[0035] It was also completely unexpected that the above-mentioneddesirable properties were achieved completely unproblematically on thebasis of the invention, and in particular even including the extensionswhich are presented below.

[0036] In an advantageous extension of the invention, it is providedthat the mirror which delimits both the main resonator and thenon-linear resonator has a transmission which is less than half, and inparticular less than ⅓, of the transmission of the extraction mirror ofthe main resonator.

[0037] The basis for this extension is that as little power as possibleshould be coupled into the non-linear resonator. If the non-linearmedium in the resonator is, for instance, an optical fibre, this couldbe destroyed by too much power being coupled into this medium. This thenrestricts the design of the laser for high average output power.

[0038] As can be seen below from calculations which are explained indetail, the stability criterion according to the invention is almostindependent of the transmission of the medium between the main resonatorand the non-linear resonator. On the basis of the invention, therefore,the power in the non-linear resonator can be kept almost arbitrarilysmall, which in turn has the consequence that the average output powercan be increased because of the lower load on the non-linear medium.

[0039] In this connection, it is particularly advantageous if the mirrorwhich delimits both the main resonator and the non-linear resonator hasa transmission less than or equal to 5%. This is a result which couldnot be expected at all on the basis of the prior art quoted above.

[0040] According to another preferred extension, it is provided that thenon-linear resonator has a medium with a non-linear index of refraction.According to the invention it was provided, as shown, that the losses inthe non-linear resonator should be as small as possible. For thispurpose it is more advantageous to use a physical process which is notessentially based on absorption, such as, in particular, mode lockingvia the non-linear index of refraction.

[0041] In a preferred extension, it is provided that the medium withnon-linear index of refraction is at least partly in the form of thecore of an optical fibre.

[0042] In this connection, the advantages of an optical fibre have beenexplained above in more detail. However, what should be emphasised inparticular here is the improved capability of the medium for adjustmentto the predetermined conditions of the laser with an optical fibre,since the phase modulation and power can be set separately via thechoice of the length and core material and/or doping.

[0043] Above all, the losses are reduced in a preferred extension of theinvention, in which at least one end of the optical fibre has ananti-reflection coating.

[0044] In another preferred extension of the invention, it is providedthat a mirror which delimits the non-linear resonator is in the form ofa mirrored end surface of the optical fibre. In this way theconstruction becomes significantly simpler and less critical. In anotherarrangement, with a large mirror of normal size, it would have beennecessary to provide a lens to focus on the core, which would requirefirstly an additional component and secondly a certain precision ofadjustment of the focus on the core. This precision could be destroyedby an impact on the laser if special actions were not taken to preventit. Such impacts cannot be excluded if a laser of the stated type isused in a large projection television set at home or if it is usedcommercially and transported from event to event.

[0045] In the following advantageous extension of the invention, aselection is made with respect to the mirror which is created bymirroring the optical fibre end. This is characterized in that only themirror which delimits the non-linear resonator and is opposite themirror which is identical to that of the main resonator is in the formof a mirrored end surface of the optical fibre.

[0046] Because the optical fibre is only mirrored on one side and thatside of the fibre which faces the main resonator remains free, theabove-mentioned advantages which result from the mirroring of theoptical fibre end are obtained, but at the other end lenses, opticalsystems or similar can be housed, allowing better coupling into theoptical fibre, to reduce the losses below the limit which is providedaccording to the invention.

[0047] In an advantageous extension of the invention, a lens forcoupling light into the optical fibre is provided, and one side of it ismirrored as a mirror for the main resonator. This not only savescomponents, but also increases the stability, since the gap from mirrorto lens automatically remains constant.

[0048] In a preferred extension of the invention, it is provided that amirror which delimits the non-linear resonator, and is not identical tothe mirror of the main resonator, has a surface which is curvedconcavely in the direction of the interior of the resonator, and thefocal distance of which is in particular less than three times and inparticular less than twice the resonator length of the non-linearresonator.

[0049] In experiments, it was established that in this way the stabilityof the intensity of the laser light could be significantly increased.This would not have been expected from the outset, because the testconstruction also had a lens between the mirror and the optical fibre.It would have been expected that instead of the stated concave mirror aflat mirror could be used, if the focal distance of the otherwiseconcave mirror surface had been taken into account in the case of thatof the lens. However, this assumption was not confirmed experimentally.On the contrary, with a concave mirror surface it has been found thatthe precision of adjustment was improved significantly, which then has afavourable effect on stability.

[0050] Another advantageous extension of the invention is characterizedin that between the outer mirrors of the main resonator or non-linearresonator further mirrors are provided to fold the light path, at leastone of them being provided to couple in the pump light. With thisextension, above all, the compactness and simplicity of the constructionare improved.

[0051] According to another preferred extension, it is provided that thelaser-active main resonator is in a form with a lasing medium betweenthe mirrors as a laser, for which two optical elements, particularly twomirrors with a curvature and a gap to the lasing medium are provided, onwhich basis the laser radiation can be emitted with a diffractionmeasuring number M²<2. Such optical elements can be mirrors or lenses.The definition of the diffraction measuring number not only allowsoutput beams of low divergence, but also makes specially good couplinginto the non-linear medium possible. In this connection, the upper limitof 2 has been shown to be specially favourable. Additionally, providingtwo mirrors to achieve a low diffraction measuring number is a speciallysimple action with which robust construction can easily be achieved.This teaching differs from known actions in the case of high-poweredlasers, where large diffraction measuring numbers are implemented.

[0052] Further advantages and special features of the invention aregiven by the following presentation of embodiments with reference to theattached drawings.

[0053]FIG. 1 shows a block diagram to explain the principle on which theinvention is based;

[0054]FIG. 2 shows a schematic drawing for the practical construction ofan embodiment;

[0055]FIG. 3 shows measured values for a pulse duration depending on amirror position;

[0056]FIG. 4 shows stability measurements on an Nd:YV0₄ laser with achange of the resonator length;

[0057]FIG. 5a, b, c show various stability measurements for differentlosses in the non-linear resonator.

[0058] The following embodiments concern devices with which mode lockingmethods are used.

[0059] The lasers which are shown are designed for ultra-short pulseswith simultaneously high average power and high repetition rate. Theoutput power and pulse duration can essentially be adjusted andoptimised independently of each other.

[0060] With the mode locking method which is used, the stability of thelaser is also sufficiently high, so that the laser system does not haveto be actively stabilised for length.

[0061] In the schematic example of FIG. 1, a main resonator 2, which isconnected via a mirror 4 of low transmission to a non-linear resonator6, can be seen. With the mirrors 8 and 10, in connection with the mirror4 in each case, two Fabry-Perot resonators are constructed. The mirror 8is also designed as an extraction mirror with a suitably chosentransmission. Contrary to other known embodiments of “APM” (additivepulse mode locking) lasers, in the case of the device of FIG. 1 theextraction from the main resonator is distributed to multiple extractionmirrors 8 and 4. The degree of extraction of these extraction mirrorscan be adjusted independently of each other, so that it is possible evenwith constant pump power to optimise the output power and the pulseduration of the laser.

[0062] By changing the degree of extraction of mirror 8, the usableoutput power can be varied. By choosing the degree of extraction ofmirror 4, the power which is injected into the coupled non-linearresonator is adjusted. Since the resulting pulse duration depends on thepower of the radiation in the coupled resonator, the pulse duration canbe varied by changing the degree of extraction of mirror 4, i.e.changing the quality of the coupled resonator. Choosing two extractionmirrors simplifies the resonator construction further, since componentssuch as a half-wave plate and a polarisation beam divider, such as areknown from the prior art for coupling non-linear resonators, are alsoomitted. Additionally, the mode locking mechanism for devices such asare shown schematically in FIG. 1 is completely independent of thepolarisation of the laser radiation.

[0063] The high passive stability of this method is achieved byminimising the losses in the non-linear resonator. In all previouslyknown constructions, the coupled resonator had low quality with highlosses.

[0064] On the other hand, with the mode locking method which is used inFIG. 1, the losses in the coupled resonator are reduced to a minimum.This means that the transmission of mirror 4 can be chosen to besufficiently small, in particular typically less than 5%. It is thuspossible to achieve that even with length changes of severalmicrometres, typically 4 to 6 micrometres, stable pulse operation ispossible. However, if the transmission of mirror 4 is too low,ultra-short pulses no longer occur, because then the power which iscoupled back into the laser becomes too low.

[0065] If the phase position of the resonators relative to each other ischanged, the high quality of the coupled non-linear resonator 6 resultsin strong intensity fluctuations in the coupled resonator. The intensityfluctuations cause a large change of the self phase modulation, i.e. thenon-linear phase in the coupled resonator. This change of the non-linearphase can compensate for the change to the linear phase within a largerange of several n, so that design interference in the main resonator isstill ensured. For this reason, with the construction which is shown andthe mode locking method which is used, stable mode-locked operation ofthe laser is possible without active stabilisation.

[0066] To characterise the losses in an optical resonator of length L,the following three magnitudes are useful:

[0067] the fineness F,

[0068] the loss coefficient α_(r), and

[0069] the photon lifetime τ_(p)=1/c·α_(r))

[0070] The fineness F is defined as usual as the ratio of the frequencyinterval to the spectral width of the modes.

[0071] The losses in optical resonators are essentially caused byabsorption, scattering in the optical components and losses because ofpartially reflecting mirrors. In the coupled non-linear resonator 6,further loss sources such as uncoated fibre end surfaces and losseswhich result from imperfect mode adjustment of the laser light to themode of the waveguide of the optical fibre can be added.

[0072] Losses V_(F) because of the optical fibre can be described by thetransmission at the single pass. The transmission T_(F) is the ratio ofthe laser power before the fibre to the laser power after the fibre,i.e. the losses can be expressed by V_(F)=1−T_(F). In the experimentallyoperated embodiments, the transmission T_(F) was typically around 75% to80%. The losses V_(F) because of the optical fibre at a single pass werecorrespondingly 20% to 25%.

[0073] On a single cycle through the resonator of length L, theintensity of the wave is reduced by the factor (1-R_(TOTAL)), where:

R _(total) =exp (31 2·α_(r) L)

[0074]$\alpha_{r} = {{\frac{1}{2\quad L}\ln \quad \left( \frac{1}{R_{total}} \right)} = {\frac{1}{2L}{\ln \left( \frac{1}{1 - V_{total}} \right)}}}$

[0075] The total loss coefficient α_(r) is therefore:

[0076] where R_(total)+V_(total)=1, if V_(total) is the total lossfactor of a cycle.

[0077] To ensure the mechanism which is used here to compensate for achange of the linear phase by the non-linear phase, the light must stillbe able to interact with itself after one cycle in the coupledresonator, i.e. it must not be weakened too much by the losses in thecoupled resonator. The condition is thus that the photon lifetimeτ_(p)=1/(c·α_(r)) must be greater than the resonator cycle timeτ_(r=)2L/c. The following then applies:

τ_(p)τ_(r)

[0078] If the above conditions are inserted into the inequality, theresult is:

1/1n(1/R _(total))<1

e

R _(total)>1/

e

V _(total)<(1−/e)≈0.63

[0079] The total losses in the coupled resonator should therefore beless than 63%, to make the desired stable mode-locked operation of thelaser possible passively, without active stabilisation. This agrees withexperimental results, as is demonstrated later.

[0080] To achieve this high quality in the coupled resonator, furtheractions can be taken in addition to the choice of a high reflectance ofthe extraction mirror 4. For instance, the coupling efficiency of thelaser radiation into the medium 12 can, if the medium 12 is an opticalfibre, be maximised by mode adjustment of the laser mode to the mode ofthe waveguide by suitable choice of a lens, which focuses the radiationinto the optical fibre.

[0081] Such mode adjustment should also be carried out for the radiationwhich is fed back from the highly reflective mirror 10 of the non-linearresonator 6 into the stated optical fibre for, for instance, thenon-linear medium. In particular, it has been shown that specially goodmode adjustment and thus optimisation of the coupling efficiency andquality of the resonator are achieved if, instead of a plane highlyreflective mirror 10, a mirror with a plano-concave surface and a focaldistance which is greater than the resonator length of the non-linearresonator 6 is used. In particular, results with a radius of curvatureof R=−2 m with a resonator length of approximately two metres have showngood results. In particular, the adjustment sensitivity of this mirror10 is then significantly reduced. Of course, as an alternative to aseparate mirror 10, the end surface of an optical fibre which is used asa non-linear medium can be mirrored.

[0082] If the construction of FIG. 1 is compared with those which areknown from the prior art, it can be seen that the number of componentsis significantly reduced with the mode locking method which is used.Additionally, this type of mode locking ensures stable operation whichdepends little on the settings of the laser and resonator parameters. Inspite of the interferometric construction, mode-locked operation isinsensitive to mechanical faults.

[0083] A construction to generate continuously mode-locked coherentradiation is presented in more detail on the basis of a device accordingto FIG. 2, which was constructed in the laboratory and on whichmeasurements were carried out. The mirrors 8, 4 and 10 have the samemeanings as in FIG. 1.

[0084] The laser crystal 14 in the main resonator 2 consisted ofNd:YVO₄. The main resonator was constructed with the mirrors 8, 20, 22,24, 26 and 4. The pump radiation from laser diodes was coupled inaccording to the arrows which are shown above mirrors 22 and 24.

[0085] Mirrors 20 to 26 are highly reflective for the emissionwavelength of the Nd:YVO₄ laser of 1064 nm. Mirrors 22 and 24 also had ahigh transmission for light of the wavelength of the pump laser diodesof 808 nm, and were given an anti-reflection coating for 808 nm on theback.

[0086] With mirrors 8 and 4, the extraction mirrors described above areimplemented with different transmission values for 1064 nm. Forinstance, mirror 8 had a transmission of 9% and mirror 4 had atransmission of 3%.

[0087] Both mirrors were given an anti-reflection coating for 1064 nm onthe back, and were vacuum deposited onto a substrate with a wedge angleof 0.5°, to prevent undesired feedback into the resonators. Likewise,the Nd:YV0₄ crystal was given an anti-reflection coating for 1064 nm and808 nm.

[0088] The radii of curvature of mirrors 20 and 26 and the distances ofthe resonator mirrors 8 and 4 were chosen so that the radiation radiusof the resonator mode in the Nd:YVO₄ crystal was adapted to theradiation radius of the pump radiation for an emission of the laserradiation with a diffraction measuring number M²<1.2. The resonator frommirror 8 to mirror 4 had a length of about 94 cm, so that a repetitionrate of the laser pulses of 160 MHz was achieved. Mirrors 20 and 26 hadradii of curvature of −500 mm. All other resonator mirrors had planesurfaces and therefore a radius of curvature of ∞.

[0089] The coupled non-linear resonator extends from mirror 4 via thedeviating mirrors 28 and 30 to mirror 10. In the coupled resonator,there is an optical fibre as a non-linear medium 12 with the geometricallength of 70 cm. The length of the coupled resonator thus correspondedto twice the length of the main resonator, i.e. 1.88 m. The length iscalculated by the known method from the optical length of the opticalfibre of dimension 0.7 m×1.45, the index of refraction of n=1.45 and thegeometrical distances of the mirrors.

[0090] As the optical fibre, a polarisation-containing optical fibrewith a mode field diameter of 7.2 μm and a numerical aperture of NA=0.11was used. This fibre had a V number of 2.05 for λ=1064 nm, and thusrepresented a single-mode optical fibre for the wavelength of 1064 nm.The coupling of the laser light into the fibre and the collimation ofthe laser light behind the fibre were carried out with lenses 32 and 34.The lens 32 can also be, in particular, plane and mirrored on the sidewhich faces the main resonator 14, so that the mirrored surface acts asa resonator mirror instead of mirror 4.

[0091] After reflection at mirror 10, the laser light returns viacomponents 34, 12, 32, 30, 28 to the extraction mirror 4, and is coupledback into the main resonator 2 again.

[0092] To adjust the length of the coupled resonator to the length ofthe main resonator, mirror 10 was fixed to an X translation table, usingwhich it was possible to adjust the length with a precision of typically10 μm. For fine adjustment of the resonator length, mirror 10 wasadditionally fixed to a piezo-electrical adjustment element, using whichthe resonator length could be adjusted to a precision within nanometres.

[0093] The pump power through two laser diodes which were coupled in thecrystal 14 via mirrors 22, 24 had a value of 2×12W=24 W. The laser then,in operation without a coupled resonator, showed an output power of 8.7W behind mirror 8. With total losses in the main resonator of 10%, thelaser in laser operation had a threshold magnification of r 34.6. Thethreshold pump power is thus 1/34.6 of the pump power which is used andwith which this main resonator was excited. With a coupled non-linearresonator, the output power behind mirror 8 was 9.3 W higher.

[0094] This laser emitted ultra-short pulses of a pulse duration of 6.8ps and a spectral width of 60 GHz. To determine the width, asech²-shaped intensity course was assumed, a curve which fitted theobserved pulse course excellently. The laser showed itself to be aboveall insensitive to changes to the resonator length, which could bedetuned continuously over a range of several micrometres, that is aphase change of several n, without the mode-locked operation of thelaser being interrupted.

[0095] The pulse duration of the laser pulses could be adjusted inparticular by the change of the resonator length of the non-linearresonator 6. For this purpose, mirror 10 was displaced using the Xtranslation table in the direction of the resonator axis over a lengthrange of almost 500 μm. In FIG. 3, the pulse duration is shown as afunction of the micrometre position, that is of the length detuning. Itcan clearly be seen that in spite of large length changes, the pulseduration changed only in a range of 6.7 ps to 10.5 ps.

[0096] If the resonator is extended over the micrometre position of 5.26mm which is applied in FIG. 3, the laser remains mode locked. However,an additional narrow peak then appears in the spectrum, in addition tothe mode-locked spectrum. The laser then emits ultra-short pulses over acontinuous background. Shortening the coupled resonator under themicrometre position of 4.8 mm which is shown on the ordinate in FIG. 3resulted in emission of a sequence of mode-locked pulses, which wasadditionally low-frequency modulated, that is the so-called relaxationoscillation occurred. The frequency with this laser system was about 400kHz, with a repetition rate of 160 MHz.

[0097] In particular, as explained above, the long-term stability of thelaser is particularly important. To determine it, the generated laserbeam was also frequency doubled, and both the fundamental and thefrequency-doubled radiation of the laser were measured using aphotodiode. Because of the quadratic dependency of the peak power of thesecond harmonic, frequency-doubled radiation is specially sensitive tochanges of the pulse duration of the laser, so that the second harmonicreacts specially critically to instability.

[0098] All measurements showed stable long-term operation of this lasersystem over three hours, in both the fundamental radiation and thesecond harmonic, although the laser was not actively stabilisedelectronically. The output power of the laser showed a variance of 0.6%in the fundamental and 0.4% in the second harmonic.

[0099] For stability, the measurement which is shown in FIG. 4, of thedependency of the average power of the fundamental and the secondharmonic of the laser as a function of the resonator length detuning ofthe coupled resonator using a piezo adjustment element, is alsoimportant. In a test, a triangular voltage with a frequency of 0.02 Hzand a maximum range of 15V was applied to the piezo element (PI 840.10,15 μm/150V). Via this voltage, a maximum change of the resonator lengthof 2.25 μm was achieved. The bottom curve in FIG. 4 shows the voltageramp, the top curve shows the average output power, and the middle curveshows the second harmonic of the laser. It can clearly be seen that thesignal of the second harmonic hardly changes, in spite of the resonatorlength change by over 2 μm. The figure shows that with this constructionand the mode locking method which is used, stable mode-locked operationof the laser is possible without active (electronic) stabilisation.

[0100] In a further experiment, the output power of the laser wasincreased. For this purpose, the pump power was increased by choosingother, more powerful laser diodes, and a different Nd:YVO₄ crystal wasused. The pump power was about 2×24 W=48 W. The output power incontinuous operation then rose to 16 W. With total losses in the mainresonator of 17%, this laser showed a threshold magnification of r=8.7.The threshold pump power is thus 1/8.7 of the pump power which is used,and with which this main resonator was excited. The output power inmode-locked operation even rose to 18.3 W. Both powers were measuredbehind the mirror 8 according to FIG. 2.

[0101] The laser continued to emit ultra-short pulses, with a pulseduration of 6.7 ps and a spectral width of 60 GHz. The peak power ofthis laser was then 17.1 kW, and thus sufficiently large to be able tooperate optically non-linear frequency conversions in an opticalparametric oscillator, or generation of higher harmonics, or totalfrequency mixing, with the highest efficiency.

[0102] The presented measurements show that with this construction andthe mode locking method which is used, ultra-short pulses of very highaverage power can be generated without active stabilisation. Above all,with the increase of the output power no deterioration of the stabilityof the laser can be observed.

[0103] The laser emitted pulses of both high average power and shortduration. With the mode locking method which is used, the outputmagnitudes of the laser system can even be optimised independently ofeach other. As can be seen from the data and the figures, the presenteddevice is above all robust against large changes of the parameters ofthe resonator and the power.

[0104] To test the computer estimate which is explained above, apolarisation beam divider and a λ/2 plate were inserted into the coupledresonator 6 between lens 34 and mirror 10. Thus by turning the linearpolarisation using the λ/2 plate, part of the light could be extractedat the polarisation beam divider, so that defined additional lossescould be introduced into the coupled resonator.

[0105] A stability measurement was then carried out for each of threedifferent total losses. For intensity measurement, the laser radiationwas steered onto a GaAsP photodiode, which is insensitive to thefundamental radiation of the laser. However, a signal was alwaysgenerated by two-photon absorption when the laser was mode locked, andcorrespondingly ultra-short pulses with high peak power were present.

[0106] The results of these experiments are shown in FIGS. 5a-e. Thebreaks in the signal which can be seen in FIGS. 5a and 5 b, at a time ofapproximately 10 s, are solely the result of calibrating the zero pointby blocking the laser beam.

[0107] In FIG. 5a, no additional losses were introduced. With atransmission of the optical fibre in the single pass of T_(F) 78% and adegree of reflection of mirror 4 of R=97%, the result for thisexperiment was a total loss of V_(total)=41%. The total losses are thusless than is required according to the estimated condition of 63%. Ascan be seen in FIG. 5a, the result is a constant signal of the diode.The laser shows stable mode-locked operation.

[0108] In FIG. 5b, the total losses are V_(total)=59%. Additionally, 30%laser power was extracted by an additional introduced beam divider. Thetotal losses of 59% are then near the estimated limit of 63%. The laseralso works significantly less stably, as can be seen in FIG. 5b.

[0109] For the measurement which is shown in FIG. 5e, the losses havebeen increased again to 68%. The total losses in this case are greaterthan the estimated limit. As the result, it can also be clearly seen inFIG. 5c that the laser now works very unstably. The laser is onlymode-locked in short intervals (maximum signal in FIG. 5c), and emittedcontinuously between them (no signal in FIG. 5c). The laser switchescontinually between these states. The operating state of FIG. 5e thuscorresponds to such conditions as are known from previous publicationsand patent specifications for free-running, not actively stabilised APMlasers.

[0110] Indicating the limit of total losses in the coupled resonatoraccording to the previous estimate is thus suitable for defining themode locking method which is used well.

[0111] In the embodiment of FIG. 2, the medium for the non-linear phaseshift is a polarisation-containing single-mode optical fibre of lengthL=70 cm. However, it is also possible to use fibres of other lengths,without the method of functioning of the device which is shown beingaffected. In the test construction described above, for instance, fibresof length 40, 50, 60 and 100 cm were used, without having to acceptdisadvantages. Fibres with which the optical length of the couplednon-linear resonator equals the length of the main resonator, that is ofwhich the length does not correspond to an integer multiple of thelatter, are particularly advantageous. More compact, stableconstructions can then be implemented. In relation to the embodiment ofFIG. 2 (repetition rate 160 MHz), this applies in particular to fibrelengths ≦50 cm.

[0112] To reduce the losses in the coupled resonator and increase thefineness, the fibre ends can also be given an anti-reflection coating.

[0113] Instead of the lens 34 and mirror 10 as in FIG. 2, the fibre endcan be mirrored directly. In this way an even lower loss is obtained,and the fineness of the coupled resonator and thus the optical stabilityof the system are increased.

[0114] It is also possible to use fibres which do not containpolarisation, but the system is then more sensitive to externaldisturbance, and the stability is less. The reason is changes of thepolarisation direction, depending on external conditions of the fibre,such as their position or bending radius, after the pulse has passedthrough the fibre. Such conditions can cause a modulation of theamplitude, above all because the Nd:YVO₄ crystal emits in polarisedform, and therefore amplifies a fixed polarisation directionpreferentially.

[0115] In addition to the use of an optical fibre, waveguides on planarstructures, such as are used in integrated optics, can be used. Theresult is further applications of the method, because materials withgreater non-linear index of refraction can also be used. In comparisonwith an optical fibre, this would result in shorter lengths of thewaveguides, so that more compact systems and even higher repetitionrates become possible.

[0116] For high powers in the resonator, volume materials can also beused. Then, for instance, elements for coupling the laser light into thenon-linear medium 12, such as the lens 32, can be omitted, resulting insignificantly reduced losses. Media which work under the Brewster angleare particularly suitable for this. Additionally, the number of mediawhich can be used is increased, giving a further adjustment and/oroptimisation of the output data of the laser.

[0117] The method can be used even if amplification media other thanNd:YVO₄ are used, because the pulse generation is caused by thenon-resonant, non-linear medium, which is independent of theamplification medium. As examples, neodymium-doped crystals in varioushost lattices, such as Nd:YLF, Nd:YAG, Nd:YVO₄, Nd:GVO₄, Nd:YPO₄,Nd:BEL, Nd:YALO, Nd:LSB, should be mentioned.

[0118] As well as the laser crystals which are doped with Nd, crystalswhich are doped with, for instance, other rare earth ions can be used.Also, mode locking and pulse shortening should be possible inconfigurations which use other laser transitions in these laser media.As well as the laser transition of the neodymium ion at 1.064 μm whichis used in the embodiment, the known laser transitions around 1.3 μm and900 nm can preferably also be excited. Mode locking using laser crystalswhich are doped with ions of the transition metals is also possible.Such laser crystals, which can be tuned over a wider wavelength range,are for instance Ti:sapphire, Cr:LiSaF and Cr:LiCaF.

[0119] The method is not restricted to a specific resonatorconfiguration. In particular, there is also the possibility of achievingmode locking with resonators of greater or less length, i.e. lower orhigher repetition rate. A special advantage resulting from a higherrepetition rate is, among other things, the smaller structural length ofsuch a system.

[0120] In the embodiment, the ratio of the lengths of the main andcoupled resonators was 1:2. Other integer length ratios of the tworesonators are also possible. For instance, a ratio of 2:3 is suitable.

[0121] This method is also not limited to the longitudinal pumparrangement of the laser diodes which is shown in the embodiments. Atransverse arrangement of the laser diodes is also possible. The lasercrystal is excited laterally by the laser diodes. This transversepumping is particularly advantageous if the spatial beam quality of thelaser diodes which are used is low, as with high-powered laser diodebars or if the laser crystal has a large absorption length. Lamp-pumpedsystems are also easily possible.

[0122] Increasing the power with this method is also possible. Aconfiguration which, for instance, includes several laser crystals inthe resonator can be chosen for this purpose. Higher pump powers of thelaser diodes are also possible.

[0123] To shorten the pulses further, the known methods of compensationfor different group speeds within the pulse, known as GVD compensation,can be applied. For this purpose, suitable optical components such ascompensation prisms, Gires-Tournois interferometers, compensationlattices or special dielectric laser mirrors can be inserted. Thesemirrors are known as “chirp mirror” or “GTI mirror”. With these mirrors,the different penetration depths of the various frequency portionswithin a pulse into the dielectric mirror layer result in the desiredcompensation of group speeds and thus a further reduction of the pulseduration.

[0124] Use of a non-linear resonator which is coupled in the way whichis presented here can also be combined with the previously known methodsof mode locking, e.g. acousto-optical and electro-optical modulation, oruse of a saturable semiconductor absorber. This contributes to a furthershortening of the generated pulses.

[0125] These explanations show that there are numerous possibilities foroptimising the presented devices for various fields of application andoutput parameters, such as power and pulse length. However, thediscussed changes always allow high robustness and stability, providedthat they conform to the criteria according to the invention.

1. Device to generate laser light, with a laser-active main resonator(2) and a coupled non-linear resonator (6), each of the two resonators(2, 6) being delimited by mirrors (8, 4; 4, 10), and one mirror (4) ofthe main resonator (2) being identical to a mirror (4) of the couplednon-linear resonator (10), characterized in that the non-linearresonator (6) is designed for a power loss less than 63% of the powerwhich is coupled via the main resonator (2) and the mirror whichdelimits the coupled non-linear resonator (6), and the laser-active mainresonator is designed so that in laser operation it has a threshold pumppower which is less than one fifth, preferably less than one tenth, ofthe pump power which is used and using which this resonator is excited.2. Device according to claim 1, characterized in that the mirror (4)which delimits both the main resonator (2) and the non-linear resonator(6) has a transmission which is less than half, and in particular lessthan ⅓, of the transmission of the extraction mirror (8) of the mainresonator (2).
 3. Device according to claim 1 or 2, characterized inthat the mirror (4) which delimits both the main resonator (2) and thenon-linear resonator (6) has a transmission less than or equal to 5%. 4.Device according to one of claims 1 to 3, characterized in that thenon-linear resonator (6) has a medium (12) with a non-linear index ofrefraction.
 5. Device according to claim 4, characterized in that themedium (12) with non-linear index of refraction is at least partly inthe form of the core of an optical fibre.
 6. Device according to claim5, characterized in that at least one end of the optical fibre has ananti-reflection coating.
 7. Device according to claim 5 or 6,characterized in that a mirror (4, 10) which delimits the non-linearresonator (6) is in the form of a mirrored end surface of the opticalfibre.
 8. Device according to claim 7, characterized in that the mirror(10) which delimits the non-linear resonator (6) and is opposite themirror (4) which is identical to that of the main resonator (2) is inthe form of a mirrored end surface of the optical fibre.
 9. Deviceaccording to one of claims 5 to 8, characterized in that a lens (32) forcoupling light into the optical fibre is provided, and one side of it ismirrored as a mirror for the main resonator.
 10. Device according to oneof claims 1 to 9, characterized in that a mirror (10) which delimits thenon-linear resonator (6), and is not identical to the mirror (4) of themain resonator (2), has a surface which is curved concavely in thedirection of the interior of the resonator, and the focal distance ofwhich is in particular less than three times and in particular less thantwice the resonator length of the non-linear resonator (6).
 11. Deviceaccording to one of claims 1 to 10, characterized in that between theouter mirrors (4, 8) of the main resonator (2) or non-linear resonator(6) further mirrors (20, 22, 24, 26) are provided to fold the lightpath, at least one of them being provided to couple in the pump light.12. Device according to one of claims 1 to 11, characterized in that themain resonator (2) is in a form with a lasing medium (14) between themirrors as a laser, for which two optical elements, particularly twomirrors (20, 26) with a curvature and a gap to the lasing medium areprovided, on which basis the laser radiation can be emitted with adiffraction measuring number M²<2.